This invention relates generally to molds, and more particularly, to an apparatus and method for molding cores for use in casting hollow parts.
Turbine systems often include a plurality of airfoils, e.g., vanes, nozzles, blades, buckets, which are hollow to provide a cooling passage. An airfoil is typically formed by an investment casting process using a ceramic core. The ceramic core is then leached out leaving a hollow passage in the airfoil.
The ceramic cores themselves are typically molded using metal molds or dies. To remove the ceramic core from the mold without damage, often a preliminary firing is required to impart sufficient strength to the core so that the core may be handled. For example, one portion or half of the mold is removed to expose an outer surface portion of the core. This exposed outer surface portion is then heated, for example, with a torch. Thereafter, the core is removed, placed in an oven, and heated.
A drawback with using metal molds to mold the ceramic cores is that the metal molds are time consuming and expensive to produce. In addition, with a new hollow part, the time and cost to develop the part increases where many design iterations of the part are involved and each design iteration requires fabrication of a new mold for molding the core.
Stereolithography is a rapid prototyping and tooling process that has become widely popular for use in rapidly producing three-dimensional solid objects directly from electronic models. For example, stereolithography has been used to produce molds for molding plastic parts. The process involves developing a solid model from a liquid photopolymer epoxy resin by exposing it to an ultraviolet laser. The resin solidifies in layers about 0.003 inch to about 0.010 inch thick with each pass of the laser.
The use of plastic tooling or molds produced by stereolithography for making ceramic cores is limited due to the requirement of a preliminary firing or torching to impart sufficient strength to the ceramic core so that the ceramic core may be handled. For example, localized temperatures experienced by the mold can be close to about 2,000 degrees Fahrenheit during this preliminary firing or torching, whereas the plastic molding is operable up to temperatures of about 400 degrees Fahrenheit. In particular, the extreme temperatures of the preliminary firing or torching results in unacceptable degradation and distortion of the plastic molds. Accordingly, there is a need in the art for an improved apparatus and method for molding a ceramic core.
An apparatus for molding a core for use in casting a hollow part in which the apparatus includes a mold having a first mold portion and a second mold portion defining a cavity for molding the core. The first mold portion includes a cutout disposed along a parting line defined between the first mold portion and the second mold portion and opening onto the cavity. Alternatively, the first mold portion includes means for molding a protective flash attached to the core and disposed along the parting line.
The first and second mold portions typically comprise a plastic material and are formed by stereolithography. The apparatus typically includes a heat shield disposable over the protective flash and parting line for preventing deterioration to the mold during a preliminary firing to strengthen the core so that the core may be handled. Advantageously, the first mold portion may include cooling tubes for enabling removal of heat from the mold during the preliminary firing.
In another aspect of the present invention, a method for molding a core from a moldable material for use in casting hollow parts includes, providing a mold as described above, providing a moldable material, introducing the moldable material into the cavity of the mold, removing the second mold portion, and heating an exposed surface of the moldable material. The method typically includes providing a heat shield and positioning the heat shield over a protective flash molded by the cutout and a parting line of the first mold portion. The method may also include providing a support, placing the core in the support, and heating the support and the core in an oven.